1) I'm currently building a product in a niche with a clear leader. Turns out it isn't that hard to steal customers from them because they appear to be rather comfortable and are unwilling to listen their customers.

2) Whatever niche you initially pick - please make the effort to do some basic customer discovery. Call some folks up who are directly experiencing the problem you are trying to solve. Even talking to 20 people will give you a lot more to go on. People are much more willing to talk if you aren't selling them anything.